The central question addressed is how estimates of IQ heritability (h2) as well as environmental effects (e2) and other parameters of interest are affected by two kinds of modifications in overidentified least squares solution systems: Crucial and plausible data perturbations (differing values for separated twin correlations, sibling and fraternal twin correlations, gene-environmental correlation, others) as well as certain plausible model modifications (varying a parameter for intrapair environmental similarity; varying the form of the model to accomodate systematic measurement error; and using different subsets of equations in a solution set). Preliminary results using a least squares routine show that varying values for intrapair environmental similarity (a model modification) affects heritability and environmental parameters (thus demonstrating their sensitivity), as do varying values for the sibling correlation (a data perturbation). Results, and proposed analyses, are contrasted with results of past studies.